The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
An engine is widely used for vehicles, ships, small generators, and the like, and attempts to increase efficiency of the engine have been made. The engine generally discharges a large quantity of heat as waste heat, and a waste heat recovery system for recovering the waste heat as energy to increase overall efficiency of the engine has been already developed.
The above-mentioned waste heat recovery system is configured to recover the waste heat of the engine as energy, convert the recovered energy into electrical energy or mechanical energy, and use the electrical energy or the mechanical energy in the engine of the vehicle, other electrical accessories, or the like.
As a representative technology of implementing the above-mentioned waste heat recovery system, there is a Rankine cycle waste heat recovery system recovering the waste heat of the engine as the energy using a Rankine cycle. The Rankine cycle includes a circulation path in which working fluid is circulated, and the circulation path of the Rankine cycle has a boiler (an evaporator) heating and evaporating the working fluid by the waste heat (heat of exhaust gas and/or heat of Exhaust Gas Recirculation (EGR) gas) of the engine, an expander expanding the working fluid supplied from the boiler to generate rotation power, a condenser condensing the working fluid discharged from the expander, and a pump circulating the working fluid in the circulation path.
The boiler is a heat source of a waste heat recovery of the engine, and examples of the boiler include an exhaust gas boiler using exhaust gas and an exhaust gas recirculation (EGR) gas boiler using EGR gas.
Here, the EGR gas has small heat flux while having a high temperature (about 300 to 400° C.), and the exhaust gas has a relatively low temperature (about 200 to 300° C.) while having high heat flux. Besides, a charge air cooler (CAC), an engine coolant, or the like has high heat flux, but has a very low temperature (about 100° C. or less), which is not suitable for the heat source.
The waste heat recovery system according to the related art is classified into 1) a structure using only the exhaust gas boiler as the heat source, 2) a structure using only the EGR gas boiler as the heat source, 3) a structure (a series connection structure) using the exhaust gas boiler and the EGR gas boiler which are connected in series with each other as the heat source, 4) a structure (parallel connection structure) using the exhaust gas boiler and the EGR gas boiler which are connected in parallel to each other as the heat source, and the like.
Among these, in the structure in which the exhaust gas boiler and the EGR gas boiler are connected in parallel to each other, waste heat recovery efficiency is relatively high. However, since this result may be varied depending on an EGR rate, it is difficult to generalize the waste heat recovery efficiency.
Particularly, a deviation of the waste heat recovery efficiency may become severe according to revolutions per minute (RPM) of the engine, a load condition, a post-processing technique (e.g., selective catalyst reduction (SCR), diesel particulate filter (DPF)), and the like. For example, the engine in which the EGR rate of a high load region is tuned to be low may also exhibit higher waste heat recovery efficiency in the series connection structure (a structure in which the EGR gas boiler and the exhaust gas boiler are connected in series with each other so that the working fluid passes through the EGR gas boiler and then passes through the exhaust gas boiler) rather than the parallel connection structure (a structure in which the exhaust gas boiler and the EGR gas boiler are connected in parallel to each other).
As such, in the waste heat recovery system according to the related art, an arrangement structure of the boilers, which are the heat sources, is made as a fixed structure. Therefore, we have discovered that since the heat sources may not be actively varied according to driving conditions, other external factors, and the like, there was a disadvantage that the waste heat recovery efficiency may not be efficiently improved.